The use of linear accelerators for the generation of either electron radiation or X-ray radiation is well known. After generating a stream of electrons, components in the radiotherapy machine can convert the electrons to X-rays, a flattening filter can broaden the X-ray beam, the beam can be shaped with a multileaf collimator, and a dose chamber can be arranged at the exit of an accelerator. A detector is mounted and is mechanically or electronically scanned synchronously with the mechanically or electronically scanned paraxial X-ray beam, providing continuous monitoring of alignment of the patient's anatomy. These systems typically provide either static fixed field radiation therapy or fully dynamic intensity modulated radiation therapy (IMRT) used by the medical community in the treatment of cancer.
One of the challenges inherent in radiotherapy treatment is the accurate positioning of the tumor in the radiation field. The main sources of the problem result from the fact that there is a natural motion of organs inside the body, which can range, for example, from approximately a millimeter in the case of the brain inside the skull, to several centimeters for the organs in the trunk above the diaphragm. Another factor relates to changes which occur in the tumor over time because of successful treatment. Over the course of treatment and as the tumor shrinks in volume, normal tissue which had been displaced returns to its original position within the treatment volume.
To accurately verify tumor positioning, detectors such as X-ray films or electronic X-ray imaging systems are commonly used in the radiation treatment diagnostic process. In the case of electronic imaging, the megavolt therapeutic X-rays emerging from the patient can be used to generate images. However, these methods at target location deliver images of low contrast and insufficient quality. As a result, imaging with megavoltage radiation is used primarily for verification, that is to confirm that the target volume has been radiated. These problems associated with utilizing high energy X-rays produced by a megavolt electron beam are the result of interacting with matter mostly due to Compton scattering, in which the probability of interactions is proportional to the electron density. Low energy X-rays typically have energies of about 125 peak kilovolts (kVp) or below, where a significant portion of the interactions with matter is photoelectric and the interactions are proportional to the cube of electron density. Low energy X-rays are more useful to provide accurate targeting or diagnostic information because tissue in the human body is typically of low density and as a result, the contrast achieved in low energy X-rays is far superior to that obtained with megavoltage X-rays. Therefore, distinctions of landmark features and the imaging of other features not perceptible with high energy X-rays are possible using kV energy. As a result, two separate imagers, each sensitive to an energy range, i.e. either the megavolt source or the kV source are used in treatment.
One method taught is to incorporate a low energy X-ray source inside the treatment head of the accelerator capable of positioning itself to be coincident with the high energy X-ray source. With this approach, a high energy X-ray target is modified to include a compact 125 kV electron gun to be mounted to a moveable flange at the base of the high energy source with the cathode of the gun operably coupled to the upstream end of a drift tube. By engaging an actuator, the electron gun can be provide target information for diagnostic imaging. An imager can be used that is sensitive to kV range radiation energies and positioned opposite the kV electron gun with the target volume in between. Therapeutic treatment can then be started or resumed by positioning the high-energy or megavolt electron beam trajectory to be in line with the target volume. A second imager is positioned opposing the megavolt source that is more sensitive to the radiation energy used in the therapeutic and verification procedure.
FIGS. 1A & 1B are illustrations of a radiotherapy clinical treatment machines to provide therapeutic and diagnostic radiation, each directed to a different imager. FIG. 1A is an illustration of the radiotherapy machine having a single diagnostic X-ray source directed to a single imager. The radiotherapy machine has a therapeutic radiation source directed to a therapeutic imager along a first axis and the diagnostic X-rays are directed to the second imager along an axis that is 90° from the first axis. This apparatus places the therapeutic radiation source capable of propagating radiation in the megavoltage (MV) energy range and the kilovoltage (kV) diagnostic radiation source on different support structures. Each radiation source has an imager opposing that is in line to the respective radiation source along an axis.
FIG. 1B is an illustration of the radiotherapy machine having dual diagnostic X-ray sources, each directed to a separate diagnostic imager. The radiotherapy machine has a therapeutic radiation source capable of propagating a therapeutic radiation beam along an axis to a therapeutic imager. Attached to support structures are two diagnostic radiation sources that can propagate diagnostic X-rays at off-angles from the therapeutic radiation axis. Each radiation source as an imager in line to receive the radiation. The entire structure of radiation sources and imagers can be pivoted together by a common base.
Cancer patients usually need to lie on their backs for radiation treatment and the patient's anatomy can shift markedly from supine to prone positions. In order to irradiate the target volume from different directions without turning the patient over, 360° rotation of the support structure holding the radiation source is needed. For convenience in setting up the patient, the isocenter around which the equipment rotates should not be too high above the floor. Adequate space must be provided between the isocenter and the radiation head for radiation technologist access to the patient and for rotation clearance around the patient. This leaves a quite limited amount of space for the various components such as the radiation shielding in the radiation head, and particularly for the magnet system. To a significant extent, the design challenge over the years has been to stay within this space, to reduce cost where possible, and while making major advances in the clinical utility of machines.